Conventional bearing units to be lubricated with grease have a grease-feeding hole (nipple). Grease-feeding is regularly carried out to recover the lubrication performance of grease and to extend the duration of use of the bearing unit. Unlike lubricating oils, grease to be fed has poor fluidity and is therefore grease-fed to a lubrication part of a bearing unit (e.g., sliding interfaces between of a rolling element and track rings, retainer, etc.) in such a manner that the grease is pressurized by a grease-feeding unit and continuously pressed into the bearing unit.
Patent Documents 1 and 2 disclose methods for feeding grease to a ball-and-roller bearing using air pressure, wherein the grease is used as it is, without diluting.
However, these grease-feeding methods do not allow regular feeding of a small amount of grease to lubrication parts. Accordingly, an excessive amount of grease is always supplied, causing an increase in rotation torque of the bearing unit and grease leakage from the bearing unit. Moreover, not all of the overly supplied grease reaches the lubrication part, and some may remain in the supply path. Thus, the whole grease does not contribute to lubrication, and generation of waste grease is inevitable. In addition, since the overly supplied grease is separated from the base oil with time, the separated oil may leak into the bearing unit. This problematically results in contamination of use environment, and curing of grease causes defects in grease-feeding.
Grease lubrication is employed, for example, in roller bearings of trolley conveyors used in automobile industries and other automation factories; however, conventional grease-feeding methods have problems, such as increase in roller rotation torque caused by overly supplied grease, contamination of use environment by grease leakage, and breakage of the roller caused by the shortage of lubrication of cured grease remaining in the supply path.
Grease having high consistency, namely soft grease, is sometimes used as feeding grease; however, such grease with high consistency has high fluidity and may therefore leak from out of the bearing unit depending on the amount of grease supplied, causing contamination of use environment.
Patent Document 3 discloses a solvent dilution-type fluorine-based lubricant composition obtained by dispersing a mixture of at least a fatty acid metal salt and a fluororesin as solid components and a base oil comprising at least a perfluoropolyether oil, in a solvent, thereby the composition preventing sedimentation of the solid components in the solvent after dilution with the solvent (e.g., a fluorine-based solvent) in a short time, and improving various properties, such as lubricity and durability.
In the solvent dilution-type fluorine-based lubricant composition, 0.1 to 20 mass % of solid components (i.e., fatty acid metal salt and fluororesin) and 0.1 to 40 mass % of perfluoropolyether oil are dispersed in the balance of 98.8 to 40 mass % of solvent. Due to this formulation, the composition can exhibit desired properties, that is, excellent dispersibility and lubrication performance. This composition comprises a fatty acid metal salt as an essential component. A composition of a comparative example, which does not use a fatty acid metal salt, comprises 1.2 to 5.0 wt. % of grease component based on 98.8 to 95.0 wt. % of solvent. The dispersibility of this composition is described to be insufficient.
Patent Document 3 indicates that the solvent dilution-type fluorine-based lubricant composition according to this invention can be effectively used to lubricate, for example, sliding parts of precision parts, the movements of which become slow because of the consistency of lubricating grease, and to lubricate between relative members to which lubricating grease is hardly supplied. Moreover, the composition is a dilution type having a lower grease content so as to be applied to thin films, and a large amount of leakage may occur during grease-feeding to a bearing unit. For this reason, the composition may not be suitable for grease-feeding.